An image forming apparatus of an electrostatic recording type comprises a print assembly including a photosensitive member (photosensitive drum), a charger, an optical head for forming an electrostatic latent image on the photosensitive member, a developing unit for converting the electrostatic latent image into a toner image and a transfer element (transfer roller) for transferring the toner image to the paper. The charger, the optical head, the developing unit and the transfer element are arranged around the photosensitive member to form a charged toner image while the photosensitive member makes one rotation. The charged toner image is transferred from the photosensitive member to the paper by the transfer element, and the paper, after the image is fixed in the fixing unit arranged behind the print assembly, is delivered.
An image forming apparatus is available which is capable of multicolor recording and comprises a plurality of print assemblies arranged in series and one fixing unit. In the case where developers of four colors including yellow, magenta, cyan and black are used, for example, the image forming apparatus comprises four print assemblies each containing one developer. The toner image formed by the print assembly of each color is superposed on the paper conveyed on a paper conveyor belt and converted into a color image.
The image forming apparatus further comprises a top cover and an end cover movably mounted on a frame. The top cover is arranged to cover a plurality of print assemblies. By opening the top cover, the component members of the print assemblies arranged in the frame can be repaired or replaced. The end cover, on the other hand, is arranged to cover the fixing unit. By opening the end cover, the fixing unit can be exposed.
Especially in a color image forming apparatus, the fixing unit is required to fix the developers of four colors, and therefore as compared with a monochromatic image forming apparatus, requires strict heating and pressing conditions. Even when the jamming of paper occurs in the fixing unit, the paper jammed in the fixing unit can be removed by opening the end cover. Once the paper has been wound around the fixing rollers, however, it is necessary to remove the fixing unit from the frame of the image forming apparatus and remove the paper from the fixing roller pair by opening the nip of the fixing rollers.
The fixing unit is secured to the frame by screws, and for the fixing unit to be removed from the frame, the screws must be removed. The removing of the screws requires a tool such as a screw driver. It is troublesome for the operator of the image forming apparatus to use a tool. Further, after removing the paper, it is necessary to mount the fixing unit on the frame and secure it on the frame by fastening the screws. In such a case, it is sometimes forgotten to tighten the screws. The job of removing the fixing unit from the frame for removing the paper wound around the fixing roller and subsequently remounting it on the frame can be comparatively troublesome.
On the other hand, the paper is conveyed to the fixing unit by a paper conveyor belt, and a paper guide the latent electrostatic image, and a transfer element for transferring the developed toner image to the paper sheet, wherein the latent image forming means is mounted on a top cover which is movably mounted on the frame to cover the print assembly.
Preferably, the nip control mechanism moves with the end cover when the end cover moves from the open position to the close position.
Preferably, the frame has a positioning pin inserted into a hole of the fixing unit for positioning the fixing unit, and the fixing unit securing means includes a lock member adapted to engage the positioning pin in cooperation with the nip control mechanism.
Preferably, the nip control mechanism includes a manually operable level, the lock member has a cam plate movable with a lever, and the positioning pin has a small-diameter portion adapted to engage the cam plate.
According to another feature of the invention, an image forming apparatus comprises a frame, at least one print assembly capable of forming an image, a pair of fixing rollers for holding and conveying a paper sheet, an inlet-side paper guide arranged to be movable vertically with respect to a paper passage in accordance with a thickness of the paper sheet for guiding the paper sheet between the fixing rollers, and a paper sheet conveyor belt for conveying the paper sheet to the fixing unit through the print assembly and the inlet-side guide.
In this configuration, the inlet-side paper guide is vertically movable, and therefore the paper is prevented from jumping up when the rear end thereof leaves the paper sheet conveyor means.
Preferably, the inlet-side paper guide is composed of an elastic member which is flexible in accordance with a thickness of the paper sheet.
Preferably, the image forming apparatus comprises means for moving the inlet-side paper guide in accordance with the kind of the paper. Most of the toner is attached and scraped off at the forward end of the cleaning felt, after which the cleaning efficiency and the service life of the cleaning felt are reduced.